In general, conventional aqueous based latex topcoats require using a primer undercoat to provide adequate adhesion and tannin blocking to wood and previously painted surfaces. Tannin blocking is the ability of a coating to prevent water-soluble chromophoric compounds, present in or on a substrate or substrate coating, from migrating through a newly applied topcoat. For example, certain woods such as redwood and cedar contain tannin staining agents. If a conventional latex coating is applied to the wood, these staining agents become extracted from the wood and migrate through the coating. Repeated applications of the same or a similar type of aqueous latex coating will not successfully prevent these stains from reappearing on the new coating surface. Solvent based coatings are typically effective in blocking these stains since the stains are not solubilized by the coating solvent.
While there are certain benefits to using a solvent based coating, aqueous coatings are preferred over solvent-based coatings for many reasons. Reduction of the VOC of the coating is an advantage offered by latex coatings. In addition, alkyd coatings are not generally as crack, fade and flake resistant as latex based coatings, particularly in exterior applications. Specially designed latex based coatings may have better adhesion to substrates such as galvanized steel and cement based substrates. Latex based coatings also offer easy water clean-up. The latex based primers currently available provide adequate stain blocking, but in order to obtain the necessary durability and attractive appearance for exterior applications, it is recommended that the latex based primer be coated with two additional layers of latex paint. The latex primer improves adhesion to the substrate and helps to insure a uniform appearance of the top coat. Primers must be top coated because they are discolored by the tannins contained in the substrate. The latex primers function by letting the tannins migrate into the primer and then trapping the tannins in the primer. This prevents subsequent latex layers from being discolored by the tannins. The additional layers of latex paint provide the durability to withstand weathering and provide a uniform, attractive appearance to the resulting coated substrate. It is desirable, however, to obtain the stain blocking properties and durability of a three layer latex coating system in a single layer of latex based coating.
The present invention is directed to a latex paint which does not require a primer for application over wood, masonry, metal, vinyl siding and previously painted surfaces. The latex paint of the present invention can be applied in a single coat directly over these surfaces to provide the tannin blocking, hiding, corrosion resistance and durability of a conventional primer plus two coats of latex paint. This primerless paint is useful for both interior and exterior applications.
The present invention is directed to a water-based coating composition including a tannin blocking latex resin, microspheres, and a reactive pigment. The microspheres have a maximum pigment loading of at least 50% and are selected from the group consisting of expanded acrylonitrile/vinylidene chloride copolymer particles, sodium potassium aluminum silicate particles, polyvinylidene chloride copolymer particles coated with calcium carbonate, ceramic particles, glass particles, and polyethylene particles. The coating composition has a PVC of 25 to 65% and a NVV of 30 to 50%. A single coat of the coating composition has exterior durability as measured by QUV testing and indicated by a xcex94E of less than 3, and tannin blocking as measured on a redwood substrate and indicated by at least a 30% improvement in the y-reflectance.
The present invention is further directed to a coated substrate wherein the foregoing coating composition is disposed over a substrate containing tannin.
The present invention is further directed to a process for coating a substrate containing tannin. The method consists essentially of applying a single coat of the foregoing coating composition to the substrate and allowing the coating composition to dry.
The present invention is directed to a latex paint which, when applied as a single coat, provides the hiding, tannin blocking, corrosion resistance, durability and application performance of a conventional paint system consisting of a primer coat plus two coats latex paint. The latex paint of the present invention comprises a tannin-blocking latex emulsion and low water absorbing microspheres.
Tannin blocking emulsions useful in the present invention include those water-borne polymer emulsions that are effective in preventing the water soluble phenolic compounds from penetrating the coating. Commercially available tannin blocking latex emulsions include Rhoplex(copyright) MV-23LO emulsion copolymer from Rohm and Haas Company which is an all acrylic composition and EPS 2532 emulsion from Engineered Polymer Solutions, Inc. which is a styrenated acrylic composition. Tannin blocking emulsions are those emulsions having a rating of at least 2 on the tannin stain blocking test described below. The tannin blocking latex emulsions may be used alone, or may be used in combination with a conventional latex emulsion. The amount of tannin blocking latex emulsion present in the coating composition is generally at least about 15% by weight of the total latex emulsion present in the coating composition.
Conventional latex emulsions include those prepared by polymerizing at least one ethylenically unsaturated monomer in water using surfactants and water soluble initiators. Typical ethylenically unsaturated monomers include vinyl monomers, acrylic monomers, allylic monomers, acrylamide monomers and mono- and dicarboxylic unsaturated acids. Vinyl esters include vinyl acetate, vinyl propionate, vinyl butyrates, vinyl isopropyl acetates, vinyl neodeconate and similar vinyl esters; vinyl halides include vinyl chloride, vinyl fluoride and vinylidene chloride; vinyl aromatic hydrocarbons include styrene, xcex1-methyl styrene, and similar lower alkyl styrenes. Acrylic monomers include monomers such as lower alkyl esters of acrylic or methacrylic acid having an alkyl ester portion containing between 1 to 12 carbon atoms as well as aromatic derivatives or acrylic and methacrylic acid. Useful acrylic monomers include, for example, acrylic and methacrylic acid, methyl acrylate, and methacrylate, ethyl acrylate and methacrylate, butyl acrylate and methacrylate, propyl acrylate and methacrylate, 2-ethyl hexyl acrylate and methacrylate, cyclohexyl acrylate and methacrylate, decyl acrylate and methacrylate, isodecylacrylate and methacrylate, and benzyl acrylate and methacrylate.
Preparation of latex compositions is well-known in the paint and coatings art. Any of the well-known free-radical emulsion polymerization techniques used to formulate latex polymer can be used in the present invention. Polymerization techniques suitable for use herein are taught in U.S. Pat. No. 5,486,576, incorporated by reference.
The addition of low water absorbing microspheres to the paint composition provides a paint composition having a high volume solids content. Useful low water absorbing microspheres include those microspheres having a maximum pigment loading of at least 50%. The Maximum Pigment Loading (MPL) percentage is an indication of the water absorption of the spherical particles. The higher the MPL, the lower the water absorption of the particles. The MPL for an extender is determined by first preparing a Base Mix as follows:
Using the Base Mix, a plurality of samples are prepared using different amounts of the extender. Each sample is prepared and evaluated as follows:
First, 300 grams of the Base Mix are added to a container having a diameter of 7 inches. A 3 inch HSD blade placed 1 inch from the bottom of the container is used for mixing the Base Mix. Next, the extender is added to the container and mixing continues for 5 minutes. The sample is then cooled to 77xc2x0 F. (xc2x13xc2x0) and the viscosity is measured using an LV Brookfield viscometer. The amount by weight of extender added is converted into a volume amount for the sample.
Using the foregoing procedure, a plurality of viscosity values for different amounts of the extender are obtained. These viscosity value are plotted vs. the gallons of dry extender per 12.65 gallons of Base Mix. From the plots, the MPL is determined. The plots result in a hyperbolic shape in which two straight lines can be drawn from the two constant slopes. The intersection of these two lines represents the point at which the mixture becomes dilatent. The x-axis reading of this point is the MPL. The MPL percentage is equal to the maximum pigment volume divided by the sum of the maximum pigment volume and Base Mix volume.
The MPL for several conventional extenders as well as several microsphere extenders are shown in Table A below.
Microspheres useful in the present invention have an MPL of at least 50%. Examples of such low water absorbing microspheres include Expancel 551 DE20, an expanded acrylonitrile/vinylidene chloride copolymer commercially available from Expancel, Inc.; Sil-Cell 35/34, sodium potassium aluminum silicate particles commercially available from Silbrico Corporation; Dualite 27 polyvinylidene chloride copolymer coated with calcium carbonate, commercially available from Pierce and Stevens Corporation; Fillite 150 ceramic spherical particles commercially available from Fillite North America, Inc.; Microbeads 4A soda lime plate glass particles, commercially available from Cataphote Inc.; Sphericel(copyright) hollow glass spheres commercially available from Potter Industries Inc.; Eccosphere FTD 235 hollow glass spheres commercially available from Emerson and Cumming; Z-Light Sphere W-1200, ceramic hollow spheres commercially available form Zeelan Industries Inc.; Scotchlite K46 glass bubbles commercially available from 3M, and Vistamer UH 1500 and Vistamer HD 1800, polyethylene particles commercially available from Composite Particle. Preferably, the microspheres have a diameter from about 1 to about 210 microns.
Pigments, such as titanium dioxide and extenders, such as zinc oxide, silicon oxide, clay, calcium carbonate, talc and nepheline syenite may be added to the paint of the present invention.
The amount of low water absorbing microspheres present in the coating composition is at least 50% by volume of the total amount of extenders present. Preferably, the microspheres make up at least about 75% by volume of the total amount of extenders present. Of the total volume of the coating composition, the low water absorbing microspheres make up at least 5% by volume.
There are paint additives that are useful in helping to control the problem of tannin stain bleed. These additives contain cations that will form lightly-colored water insoluble compounds with the tannate ions to prevent their migration through the paint film. Reactive pigments, which include base pigments such as wollastonite, talc or mica in combination with phosphate or borate of Ca or Zn, and as a doping agent or active additive, one amphoteric metal hydrate of Al, Ti, Zr, Zn or Si may be added to the paint composition. A preferred reactive pigment is calcium barium phosphosilicate, commercially available as Halox BW-100 from Halox Pigments. Other reactive pigments that may be used in the paint composition include calcium phosphosilicate and aluminum zirconium phosphosilicate.
A complexing agent for a transition metal ion may also be included in the paint composition. A preferred complexing agent is a salt of phosphonic acid commercially available as Bubond 357 from Buckman Laboratories.
Extenders useful in helping to prevent the migration of tannate ions include calcium carbonate, nepheline syenite, talc and zinc oxide. The pigments and extenders present in the paint formulation contribute to the in tannin blocking characteristics in the paint film.
The relationship between durability of the coating and the amount of pigment is represented by pigment volume concentration (PVC), which is the fractional volume of pigment in a unit volume of resin. Thus, low PVC coatings, such as semi-gloss paints, contain relatively low levels of pigment, and high PVC coating compositions, such as satin to flat paints, contain high levels of pigments. The PVC of the coating composition of the present invention is generally within the range of 25 to 65%, and preferably about 40%. The solids content by volume (NVV) of the coating composition is generally within the range 30-50%, and preferably at least 40%.
The latex paint of the present invention is prepared according to standard manufacturing techniques. Such techniques involve the preparation of a polymeric binder or binders, mixing of component materials, dispersing of pigments, and a thinning adjustment to commercial standards. High speed dispersers or dissolvers are typically used in the dispersing step to intersperse the pigments into a liquid phase containing other paint components. The binder and pigment dispersions can be thoroughly and uniformly mixed with raw batch ingredients by homogenizing the binders, pigments, plasticizers and other components to form a uniform blend. The polymeric binder further maintains the pigment in stable dispersion. Although special equipment conventionally used to produce high solid systems or incorporate microspheres can be employed, this invention requires no modification to any conventional latex paint manufacturing process.